The light-sensitive wavelength range of silver halide is in the range of up to blue light, for example, from ultraviolet to 480 m.mu. in case of silver bromide, which is very different from the wavelength range visible to human eyes. In the field of this art, the sensitivity of silver halide itself (namely, the sensitivity to light up to blue light) is called the intrinsic sensitivity of silver halide.
As a means for extending the sensitivity of silver halide to the longer wavelength range, spectral sensitization techniques using sensitizing dyes are well known in the art, and many studies thereon have been done.
Spectral sensitization techniques using sensitizing dyes are particularly necessary for color sensitive materials as a means of extending the light-sensitive range of the sensitive materials. However, they have the defect of reducing the intrinsic sensitivity of silver halide, though the light sensitivity range is extended.
Therefore, methods of carrying out effective spectral sensitization while preventing a reduction of the intrinsic sensitivity have been much desired.
For example, a method in which the amount of the sensitizing dye is reduced as much as possible in order to prevent a reduction of intrinsic sensitivity is known, but it has the defect that spectral sensitization is insufficient because the amount of the sensitizing dye is too small.
Further, techniques for improving the efficiency of spectral sensitization using dyes having a specific structure as sensitizing dyes have been utilized, but the effect of spectral sensitization itself is still insufficient. Further, since they have defects that they cause a deterioration of silver halide sensitive materials with the passage of time during storage and latent images formed on silver halide fade during the period from exposure to development processing, they are not always satisfactory.
In recent years, it has been highly desired that silver halide sensitive materials have higher sensitivity, and it has been highly desired in this art to develop a method of effectively carrying out spectral sensitization.